The invention pertains to a device for testing hardness under load. It has at least one measuring sensor in the form of a manual apparatus. This manual apparatus has a pencil-like housing in which a mobile rod-shaped resonator can be slidingly inserted. The resonator has
(a) on its free end, which is situated outside the housing, a measuring tip;
(b) and the resonator is held against the housing of the measuring sensor by a spring.
(c) The resonator is linked to ultrasonic converters that are connected to an electronic generator or to a receiving circuit that records changes in the duration of frequency of the vibrations of the generator.
The device incorporates also a basic apparatus that comprises a display unit and is connected by a connecting line to the measuring sensor.
In the resonance process for testing hardness which is familiar from U.S. Pat. No. 3,572,097 frequently designated the UCI-(Ultrasonic Contact Impedance)--process, a rod-shaped resonator is activated longitudinally with the help of piezo-electric ultrasonic converters to produce vibrations. In this device a rod with the dimension ratio 1.times.1/4 is employed, thereby ensuring that only the first overtone can be formed. The rod is fitted on its lower, free end with a penetrating device that is manufactured of diamond. When this system is coupled elastically to a mass or body the resonating frequency of the resonator is modified to higher values because an additional repulsion (resetting) force acts on the resonating system. The value of this repulsion force and with it the modification to which attention has been drawn are dependent on the surface of contact between the penetrating body and the body to be tested and the elastic property of this surface.
In each instance the variation in frequency is a function of the surface of contact, the elasticity modulations of the diamond or, respectively, of the body to be tested, the Poisson's ratio of the diamond or, respectively, of the body to be tested and the resonating frequency of the resonator. If the elasticity modulations and Poisson's ratio are not known the testing device of the familiar kind can be calibrated by means of a comparison (calibration) plate of the kind that is already well known. Thus it is possible to carry out measurements on bodies whose elastic constants vary just a small amount from those of the calibration plate, it being necessary in instances where the resonance frequency is known to measure only the frequency variation. In practice it has been found that non-alloyed steels and steels with a low alloy have to a very large extent overlapping elasticity modulations or, respectively, Poisson's ratios; the calibrations process is therefore particularly suited for steels of this kind. The apparatus for testing hardness under load of the kind to which attention was drawn above is familiar from the journal article "VDI-Report Number 583/1986, pp. 371-391". In this device, which is designed predominantly for testing small loads, all the electronic parts but especially the electronic generator, which is designed to stimulate the resonator, and the electronic receiving circuit are sited in the analyzer. Just one sensor is attached to each analyzer, and sensor and analyzer are attuned to each other. Thus it is possible to exchange one sensor for another only if the analyzer is calibrated anew each time. This is a disadvantage. In general, when measuring under various testing pressures several devices for measuring hardness are employed because too much time is required to calibrate anew the sensors. On the other hand, attempts to manufacture the sensors so accurately that one can be exchanged for another have shown that variations which always result during manufacture can be avoided only at unusually huge expense, for which reason it is not possible to achieve in this way an economic solution to this problem.
In the apparatus for testing hardness of the kind mentioned above the spring that is sited between the rod-shaped resonator and the housing for the measuring senso is designed as a tension roll spring. Such springs are, on the one hand, bulky, and on the other hand, expensive. The tension roll spring determines essentially the dimensions of the measuring sensor.
The German Patent Document 33 29 690 describes a process and a device for simplifying the testing procedure in accord with the Contact Impedance Method. In this process the vibrations of the freely resonating rod-shaped resonator are measured over an initial or first period of time, and then, in a second time period that is of the same duration, the vibrations of the resonator, which is now connected to a test-piece, is measured. During the second time period the vibrations of the resonator that is coupled to the test piece are counted only until zero value is achieved in the subtraction of this value and the value that was ascertained during the vibrations in the first time period; during the time remaining in the second time period the vibrations are counted over strictly delimited period. In one version of the device the length of the first time period is set at 355 ms. The German Patent Document 35 04 535 describes for determining the hardness of solid bodies a process that essentially is completed with a device such as is outlined in the above-mentioned VDI-report. In this process, however, no differences in frequency are measured; rather, the process determines the amplitude of the rod-shaped resonator when it is mechanically coupled with the body to be tested, and from the square of the amplitude values the hardness of the subject body can be calculated.